Field of the Invention
The following disclosure describes a method and system for determining the position of a mobile radio operating in the cellular radio service. A position service would have many desirable applications in the cellular radio service such as, location service for emergency callers (911), child locators, dispatch services, and fleet monitoring systems. Also, cellular system operators could use such methods to customize service parameters based on accurate knowledge of mobile telephone location such as lower cost services for limited mobility customers. Such a service would also be of use in locating stolen cellular phones and for investigating fraudulent use of cellular services.
Methods for radio position determination make use of techniques for measuring the propagation delay of a radio signal which is assumed to travel in a straight line from a transmitter to a receiver. A radio delay measurement in combination with an angle measurement provided by a directive antenna is the fundamental principle of radar. Radar location is frequently augmented by use of a transponder in the mobile vehicle rather than relying entirely on the signal reflected by the mobile vehicle.
Alternatively, multiple time delay measurements can be made using multiple transmitters and/or receivers to form a so-called tri-ateration system. The Loran system is an example of a system which transmits a series of coded pulses from base stations at know and fixed locations to mobile receivers. The mobile receiver compares the times of arrival of signals from the different transmitters to determine hyperbolic lines of position. Similarly, the global positioning system (GPS) provides transmission from a set of 24 earth orbiting satellites. Mobile receivers can determine their position by using knowledge of the satellites'locations and the time delay differences between signals received from four or more satellites.
From the above examples, it can be seen that radio position location systems can be divided into two types, those which allow a mobile user to determine its own position and those which allow another party to determine the position of a mobile transponder such as radar systems. The system herein disclosed is of the second type where the fixed portion of a cellular telephone system determines the location of a mobile cellular telephone. Generally, such systems require that the mobile user transmit a radio signal (except in the case of passive radar.)
Methods of radio location such as disclosed in "Dual Satellite Navigation Method and System." U.S. Pat. No. 5,126,748, issued June, 1992 require the mobile terminal to both transmit and to receive which allows round trip timing measurements defining circular lines of position to be performed using fewer transmitter sites than required for the Loran and GPS systems in which the mobile terminals contain only receiving capability. In other systems, the mobile terminal may contain only a transmitter and the remaining system elements perform direction finding or multiple receptions of the signal from different locations to determine the position. An example of this is the SARSAT system for locating downed aircraft. In this system, the downed aircraft transmits a signal on the international distress frequency 121.5 MHz (and 273 MHz). An earth orbiting satellite relays the signal to an earth terminal. As the satellite passes overhead, the change in Doppler shift can be detected and a line of position can be determined. Multiple overhead passes by the same or similar satellites can determine a set of lines of position, the intersection of which determines the location of the downed aircraft.
In the disclosed system, we wish to make use of the existing capabilities of mobile cellular telephones operating in the AMPS service (or similar service) to provide a new service of position location without modifying the millions of already existing AMPS mobile cellular telephones. In the AMPS service, the mobiles transmit at UHF frequencies between 824-849 MHz and base stations transmit at frequencies between 869-894 MHz. The frequency bands are divided into two sets of 833 channel pairs evenly spaced 30 kHz apart. One set of 833 channel pairs is licensed to each of two service providers in a given area.
The AMPS system uses analog FM modulation to transmit telephone speech. The mobile and base stations transmit simultaneously using full duplex techniques so that the user perceives a continuous link in both directions at all times.
Normally, each base station in a large cellular system serving a metropolitan area will be assigned a set of 57 channel pairs for providing telephone service. Additionally, one channel is assigned for signaling and paging. Calls are initiated at the mobile station by transmitting a digital message to the nearest base station on its control channel. The base station will respond on its corresponding control channel with a channel assignment to be used by the mobile while the call is in progress within the confines of this cell. If the call continues while the mobile moves into another cell, a control message from the base station will command the mobile to change channels to one assigned to the cell the mobile is moving into in a process called handoff.
The AMPS system includes a technique called supervisory audio tone (SAT) to insure that calls are being handled by the proper base stations. In this system, each base station adds a high frequency audio tone to the telephone audio of each call in progress. This tone will either be transmitted at 5980, 6000, or 6030 Hz. The mobile station will detect and filter this tone and transmit it back to the base station by adding it to the telephone audio. The base station then filters and detects the SAT tone and insures that the received tone is the same frequency as the tone it transmits. A pattern of SAT tone assignment to different neighboring base stations allows instances of incorrect connections to be detected and corrected.
When the AMPS system was originally being defined, it was contemplated that the mobile stations'positions could be located by measuring the phase difference between the forward link SAT tone and the SAT tone received by the base station from the mobile. This would permit a round trip time delay measurement which would locate the mobile on a circle around the base station. It was seen that this technique would introduce the need for a specification controlling the phase shift of the returned SAT tone in order to provide consistent measurements. Because of this added complexity, this approach was dropped from the specification.